In order to measure the bolt preload with acoustoelasticity accurately under the low voltage excitation, a signal processing method based on correlation operation is utilized to process the echo signal. In this method, the ultrasonic propagation time is obtained by comparing the most similar position of the transmitted wave in the echo, so as to measure the preload. The correlation operation can suppress the noise, which can realize the measurement of transmission time under the condition of low SNR. At the same time, the method combined with interpolation can improve the resolution of time measurement. The Linear Frequency Modulation(LFM) pulse is chosen as the emission wave to test the preload of the bolt using acoustoelasticity under different voltage excitation. The maximum error is below 2.9%. The test results show that the method can realize the accurate measurement of ultrasonic propagation time under the condition of low voltage excitation, and the resolution of time measurement can be improved by combining the interpolation method.